This proposal seeks to obtain data which will explain the reasons underlying a lower extraction ratio and secretory capacity (Tm) for p-amino-hippurate (PAH) in the neonate as compared to the adult. The structural changes which occur in the developing kidney will be examined in order to correlate the maturation of the renal secretory capacity for PAH with concurrent vascular events. Specific attention will be directed to the proximal tubule and its associated peritubular capillaries; both qualitative and quantitative observations will be obtained by means of Microfil casts, light microscopy and transmission and scanning electron microscopy. As development proceeds it will be possible to determine (1) if pre- and post-glomerular shunts exist in the neonate and (2) if there is a differential maturation of the nephrons and associated vasculature which manifests itself as individual or regional vascular tubular heterogeneity.